The present invention relates to a drum brake system and more particularly to a retraction spring operative with the brake shoes to resiliently hold the brake shoes in a deactuated position disengaged from the braking surface on the brake drum and upon brake release after actuation to return the brake shoes to the deactuated position out of engagement with the braking surface.
In a conventional drum brake system for automotive applications a pair of brake shoes can be actuated by the vehicle operator to move from a radially inward disengaged position to a radially outward engaged position against a cylindrical braking surface on a brake drum. Upon release or discontinuance of the actuation by the vehicle operator the brake shoes are automatically disengaged from the braking surface of the brake drum and returned radially inwardly to the deactuated position by the bias exerted by a return spring. In prior drum brake systems the return spring has been a coil spring connected between the two brake shoes to bias the shoes radially inwardly. See for example U.S. Pat. No. 2,616,528 issued Nov. 4, 1952 to H. E. Swanson for "Brake Shoe With Removable Lining". More recently, however, drum brake systems have been designed utilizing a generally planar wire return spring in place of the coil spring. Such wire springs have been generally U-shaped and constructed from a wire having a circular cross-section. Such wire springs are generally of a type shown in U.S. Pat. No. 4,762,209 issued Aug. 9, 1988 to Copp for "Drum Brake Assembly And Shoe Hold-Down And Retraction Spring Therefor". While the wire spring in the '209 Copp patent is shown having an additional twist or turn in its end section, wire springs having a substantially purely U-shape are also known in the art. All of such U-shaped wire return springs, however, have a pair of substantially straight spring arms extending linearly from opposite sides of an arcuate end section.
Because of the geometry of such prior wire springs their effective spring rate and load bearing capacity are limited. In the present invention, a wire return spring having a unique contour and cross-section is utilized whereby higher spring rates can be provided and thus heavier loads can be sustained with substantially little increase in overall size in comparison to the prior wire return springs having a U-shaped contour and which are made from a wire having a circular cross-sectional area.
In the present invention, a wire return spring is formed to have a generally overall arcuate or truncated elliptical shape in its unassembled, free condition. As will be seen, this unique wire spring has a generally circular end section with a pair of arms extending arcuately from opposite ends of the end section. In assembly to the brake shoes the outer ends of the arcuate arms of the wire spring are first moved, resiliently radially outwardly from a neutral position to an activated assembled position such that upon release after assembly to the brake shoes a radially inwardly directed bias is applied to the brake shoes to hold them in their deactuated or return position away from the braking surface of the brake drum. In this assembled condition with the brake shoes held in the return position the wire spring assumes a generally more overall circular contour. In addition, the wire return spring of the present invention is constructed from a wire having a rectangular cross-section which, in one form, is essentially a square section. It is believed that the unassembled overall arcuate or truncated elliptical shape of the wire spring and its more circular shape when assembled plus the rectangular cross-section provides the wire spring of the present invention with a higher spring rate and hence greater load capability when compared to prior U-shaped springs of comparable size. In addition the more circular contour assumed by the wire spring upon assembly permits it to be more readily fitted relative to the other components of the brake system having a circular contour or orientation.
While the embodiment of the wire return spring of the present invention is shown and described in conjunction with a manually actuated drum brake system for a vehicle parking brake it should be understood that the wire spring can also be used in drum brake systems for moving vehicle applications while driving and including drum brake systems that are hydraulically or pneumatically actuated.
Therefore it is an object of the present invention to provide a wire type return spring for a drum brake system with the wire spring having a unique contour and cross section.
It is another object of the present invention to provide a wire type return spring for a drum brake system with the wire spring having a generally overall arcuate or truncated elliptical shape in its unassembled condition.
It is still another object of the present invention to provide a wire type return spring for a drum brake system with the wire spring having a generally overall arcuate or truncated elliptical shape in its unassembled condition and having a generally more overall circular shape when assembled to the related brake shoes.
It is another object of the present invention to provide a wire type return spring for a drum brake system with the wire spring having a generally overall arcuate or truncated elliptical shape in its unassembled condition and made of a wire having a generally rectangular cross-section.
It is another object of the present invention to provide a wire type return spring for a drum brake system with the wire spring having a generally overall arcuate or truncated elliptical shape in its unassembled condition and having a generally more overall circular shape when assembled to the related brake shoes and with the wire spring having a generally rectangular cross-section.